A possible biological interpretation is that the cells are optimizing (regulating) the environmentally relevant features (e.g. the \(z\) /\(u_g\)  yield).
That is, the culture observable state is an optimum, but only in the environmentally relevant (\(z,~u_g\) ) subspace (because the glucose-limited condition).
Once achieved such optimum, there are no further regulatory constraint affecting the degenerated dimensions.
An extra detail related with the experimental conditions, which supports such rationale, is that the studied cultures were run at small dilution rates (\(D<0.5~h^{-1}\) ).
This locates the cultures in a regime of slow growth rate (wild \(E\ coli\)  can growth at \(>2.2~h^{-1}\)  [1]}) and below the acetate switch [2]}.
This is relevant because the lower the growth rate, the less pressure is exerted on the cellular resources, and thus, internal regulations such as enzyme cost constraint (see Equation (REF )) lose significance [3]}, [2]}, [5]}.
On the other hand, this also could be an explanation for the poor performance of \(FBA\)  lineal formulations (see Figure (REF )).
\(FBA\)  predicts the optimum at the (\(z\) , \(u_g\) ) subspace, but there are no reason for the other dimensions to be also in an optimum.
This is the case, for example, of the known common ill-prediction of the flux through the glyoxylate cycle.
\(FBA\)  lineal formulations typically infer a zero flux (an optimum) where experiments report a non-zero value [6]}, [7]}.
